The present invention relates to a device for spinal osteosynthesis, comprising a strengthened rod with three branches and screws or hooks with cylindrical heads, said rod being arranged in the bottoms of longitudinal grooves of said heads and fixed by screws or nuts.
It is known that rods of circular cross section are generally applied for the spinal ostehosynthesis. The roughness of the surface of such bars is generally increased by serration or indentation. The bars are cooperating with screws or hooks to be implanted, the heads of which are constructed to receive said rod and to be mutually blocked by a nut or screw.
It is also known to connect two vertebrae to each other or one vertebra to the sacrum with plates provided with orificies and screws being screwed into the vertebrae and/or to the sacrum through said orifices.
In the first case, the cross-section of the bar is determined in a way to obtain a certain rigidity and, at the same time, to allow a transverse deformation which enables to compensate some disalignments.
In the second case, the plate has a thickness to allow some deformation in transverse direction meanwhile it has a great rigidity in the lateral direction.
Due to the above circumstances, the rods can change their shape after having been implanted, while the plates are extremely rigid in that direction. Accordingly, no deformation can take place in this respect, which requires, however, a rather high precision in positioning the screws.